Fatty acyl isethionates (e.g., cocoyl isethionates) are anionic surfactants highly desirable in personal care skin or hair cleansing products, particularly in personal care products, because they lather well, are mild to the skin and have good emollient properties. Typically, fatty acid isethionates are produced by esterification of fatty acids or by reaction of fatty acid chloride having carbon chain length of C8 to C20 with isethionate. A typical fatty acyl isethionate surfactant product (e.g., commercially sold or made surfactant product) contains about 40 to 95 wt. % fatty acyl isethionate, and 0 to 50 wt. %, typically 5 to 40 wt. % free fatty acid, in addition to isethionate salts, typically at less than 5%, and trace (less than 2 wt. %) of other impurities
A problem with the ready use of commercially sold or made fatty acyl isethionate product in liquid compositions, especially wherein the acyl isethionate surfactant product is used as a primary surfactant, is the low solubility of these compounds in water. This is especially true for fatty acyl isethionate surfactant product containing high level of free fatty acid/fatty soaps (10% by wt. or higher) and/or long chain fatty acyl isethionates component with carbon chain length 14 and higher. The fatty acyl isethionate component of the surfactant product tends to form insoluble surfactant crystals with the amount of crystals depending strongly on the storage temperature due to the wide range of dissolution temperatures of these crystals. This in turn results in unstable liquid cleansers which exhibit very thick or very thin consistency at low and elevated temperatures. At low temperature, the liquid composition becomes a semi-solid gel which is difficult to use. At elevated temperature, the liquid composition turns into water-thin liquid which causes phase separation of the product. This is exemplified in FIG. 1 in which the viscosity at 25 and 40° C. of Comparative Example 1A is shown. Comparative Example 1A is a liquid which contains 36 wt % of fatty acyl isethionate product (Dove® Noodle) and has a viscosity ratio at 40° C., to 25° C., of about 0.0234 when measured at 0.01 s−1. At 25° C., this liquid has very high viscosity; and the liquid composition becomes water thin at 40° C.
It would therefore be of tremendous advantage to have compositions, particularly those where fatty acyl isethionate surfactant product with 10 wt % of more free fatty acids is used, having consistent viscosity at both low and elevated temperatures. It would further be advantageous to have a way of manipulating compositional ingredients to ensure such consistent viscosity is obtained and that fatty acyl isethionate product, no matter what their free fatty acid/fatty soap content or what the chain lengths of the fatty acyl isethionate, fatty acid or fatty acid/fatty soap component, can be readily used in a liquid cleanser composition. The present invention provides precisely such compositions and processes for making such compositions.
While not wishing to be bound by theory, applicants believe the problem of inconsistent viscosity and physical instability at elevated temperatures (i.e. 40° C. or higher) for these liquid compositions is caused because the compositions contain insoluble fatty acid/fatty acyl isethionate surfactant crystals at room temperature. Applicants believe the noted stability problems can be resolved by forming viscous surfactant liquid crystals at a temperature at or above the dissolution temperature of these fatty acid/fatty acyl isethionate crystals such that the liquid composition in which the surfactant product will be used has a high enough viscosity to ensure stability, said stability being defined by the absence of visible physical separation after two weeks of storage at 45° C. Formation of such viscous crystals in turn is accomplished by using specific mixture of long chain (>14) and short chain (≦C14) linear fatty acids/fatty soaps (i.e., >20, but less than 70 wt. % of total fatty acids/fatty soaps is long chain fatty acids) in the liquid compositions to create a higher viscosity at elevated storage temperatures (more consistent viscosity) which allows fatty acyl isethionate product, regardless of free fatty acid content or the chain length of isethionates, to have more consistent viscosity at low and elevated temperatures and to be storage stable.
Acyl isethionate liquids do exist in the art. U.S. Pat. No. 5,415,810 to Lee et al., for example, discloses compositions comprising fatty acyl isethionates and zwitterionic surfactant (e.g., cocoamidopropyl betaines), presumably to help solubilize the isethionate and make an isotropic liquid. The reference separately teaches away use of free fatty acids, especially longer chain fatty acids (column 2, lines 34-39), let alone the use of both long chain and short chain fatty acids in combination.
U.S. Pat. No. 5,739,365 to Brody et al. and U.S. Publication No. 2004/0224863 both disclose use of synthetic surfactants having ammonium counterion to help solubilize fatty acid isethionate.
U.S. Pat. No. 5,952,286 and U.S. Pat. No. 6,077,816, both to Puvvada, disclose liquid cleansing compositions which may contain acyl isethionates and which comprise soluble, lamellar phase inducing structurant (e.g., branched fatty acid).
U.S. Pat. No. 5,132,037 to Greene et al. (and related U.S. Pat. No. 5,234,619 and U.S. Pat. No. 5,290,471) disclose compositions with C8 to C22 acyl isethionates, synthetics, and free fatty acids, preferably C16 or higher. Free fatty acids of 8-22 carbons are incorporated within the compositions claimed in these patents to operate as lather enhancing agents and as skin feel, lubricity, and creaminess enhancers rather than as stabilizer to maintain liquid consistency at low and high temperature storage conditions (lines 29 to 39, column 2 of U.S. Pat. No. 5,132,037). There is no teaching on how to use a specific mixture and the required level of long chain and short chain fatty acids to provide consistent viscosity of liquid composition containing fatty acyl isethionate surfactant products. These patents broadly claimed fatty acid composition (C8 to C22) and its level (1/10 to 10/1 of fatty acids to fatty acyl isethionate surfactants) and are silent to the problem of product inconsistency and storage stability at high temperature for liquid compositions containing insoluble fatty acids/fatty acyl isethionate surfactant crystals. All the comparative examples of the subject invention with compositions as claimed in these patents (Table 3 below) show these liquids of Greene et al. are not stable at 45° C. These comparative examples all showed phase separation in less than 2 weeks at 45° C. storage condition. Compositions that can provide consistent viscosity at elevated temperature stability are not obvious or predictable based on a broad disclosure of fatty acid and fatty acid/surfactant ratio in these patents. It would not be a function of product optimization either since the previous art would not have been aware of what caused the problem and it is completely unpredictable that this is an approach that could work.
Applicants filed in December 2006 three cases relating to liquid compositions with crystal modifier systems similar to those of the subject invention. It would not have been predictable in any of these applications, however, that the specific level of long chain and short chain fatty acids/fatty soaps mixture alone could be used to stabilize liquid compositions containing fatty acyl isethionate surfactant product, e.g., by forming viscous surfactant liquid crystals upon the dissolution of fatty acyl isethionate/fatty acid crystals at elevated temperatures (40° C. or above).
None of the references, alone or together, teach or suggest compositions comprising fatty acyl isethionate surfactant containing specific long chain and short fatty acid/fatty soaps mixture (20 to 70%>C14 and 80 to 30%≦C14) where and the compositions formed have a product viscosity less sensitive to temperature, and are stable at elevated temperature storage conditions.